Crystal laminate structure

ABSTRACT

A crystal laminate structure includes a Ga 2 O 3 -based substrate, and a β-Ga 2 O 3 -based single crystal film formed by epitaxial crystal growth on a principal surface of the Ga 2 O 3 -based substrate. The β-Ga 2 O 3 -based single crystal film includes Cl and a dopant doped in parallel with the crystal growth at a concentration of not less than 1×10 13  atoms/cm 3  and not more than 5.0×10 20  atoms/cm 3 .

TECHNICAL FIELD

The invention relates to a crystal laminate structure.

BACKGROUND ART

Conventionally, a method in which a dopant is added during crystal growth by the MBE (Molecular Beam Epitaxy) method or the EFG (Edge-defined Film-fed Growth) method (see, e.g., PTLs 1 and 2) and a method in which a dopant is added to a grown β-Ga₂O₃-based single crystal by ion implantation (see, e.g., PTL 3) are known to dope a β-Ga₂O₃-based single crystal.

CITATION LIST Patent Literature [PTL 1] JP-A-2013-56803 [PTL 2] JP-A-2013-82587 [PTL 3] JP-A-2013-58637 SUMMARY OF INVENTION Technical Problem

When using the MBE method, however, doping with a concentration of not less than 1×10¹⁸ atoms/cm³ is difficult since impurities segregate during epitaxial crystal growth. In addition, a doping profile in a depth direction and distribution uniformity on the wafer surface are not good due to the impurity segregation and repeatability is poor. Meanwhile, when using the EFG method, since a raw material contains an impurity at a concentration of about 1×10¹⁷ atoms/cm³, doping with a lower concentration is not possible.

In case of ion implantation, implantation depth of the impurity ion is limited to about 1 μm. In addition, since the ion beam damages the crystal, vacancy/void-type defects are introduced, causing deterioration of crystallinity.

Therefore, it is one of objects of the invention to provide a crystal laminate structure having a β-Ga₂O₃-based single crystal film in which a dopant is included throughout the crystal and the concentration of the dopant can be set across a broad range.

Solution to Problem

To achieve the above-mentioned object, an aspect of the invention provides a crystal laminate structure defined by [1] to [10] below.

[1] A crystal laminate structure, comprising: a Ga₂O₃-based substrate; and a β-Ga₂O₃-based single crystal film formed by epitaxial crystal growth on a principal surface of the Ga₂O₃-based substrate, wherein the β-Ga₂O₃-based single crystal film comprises Cl and a dopant doped in parallel with the crystal growth at a concentration of not less than 1×10¹³ atoms/cm³ and not more than 5.0×10²⁰ atoms/cm³.

[2] The crystal laminate structure defined by [1], wherein a concentration of Cl of the β-Ga₂O₃-based single crystal film is not more than 5×10¹⁶ atoms/cm³.

[3] The crystal laminate structure defined by [1] or [2], wherein the dopant comprises Si.

[4] The crystal laminate structure defined by [1] or [2], wherein the dopant concentration is not less than 6.5×10¹⁵ atoms/cm³ and not more than 2.1×10²⁰ atoms/cm³.

[5] The crystal laminate structure defined by [1] or [2], wherein a density of a carrier generated by the doping of the dopant is not less than 1×10¹³ cm⁻³ and not more than 5.0×10²⁰ cm⁻³.

[6] The crystal laminate structure defined by [5], wherein a density of a carrier generated by the doping of the dopant is not less than 3.2×10¹⁵ cm⁻³ and not more than 1.8×10¹⁸ cm⁻³.

[7] The crystal laminate structure defined by [1] or [2], wherein the β-Ga₂O₃-based single crystal film comprises a β-Ga₂O₃ crystal film.

[8] The crystal laminate structure defined by [1] or [2], wherein the principal surface of the Ga₂O₃-based substrate has a plane orientation of (001), (010), (−201) or (101).

[9] The crystal laminate structure defined by [3], wherein a source gas of the dopant comprises a SiCl₄ gas.

[10] The crystal laminate structure defined by [1] or [2], wherein a thickness of the β-Ga₂O₃-based single crystal film is not less than 1000 nm.

Advantageous Effects of Invention

According to the invention, it is possible to provide a crystal laminate structure having a β-Ga₂O₃-based single crystal film in which a dopant is included throughout the crystal and the concentration of the dopant can be set across a broad range.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a vertical cross-sectional view showing a crystal laminate structure in an embodiment.

FIG. 2 is a vertical cross-sectional view showing a vapor phase deposition system in the embodiment.

FIG. 3 is a graph which is obtained by thermal equilibrium calculation and shows a relation between equilibrium partial pressure of GaCl and an O₂/GaCl supplied partial pressure ratio when the atmosphere temperature during Ga₂O₃ crystal growth is 1000° C.

FIG. 4 is a graph showing a relation between R_(Si) and Si concentration of a grown β-Ga₂O₃-based single crystal film.

FIG. 5 is a graph showing a relation between R_(Si) and carrier density of the grown β-Ga₂O₃-based single crystal film.

FIG. 6 is a SIMS profile of Sn in a sample c4 shown in Table 3.

FIG. 7A is a SIMS profile of Si in a sample c3 shown in Table 3.

FIG. 7B is a SIMS profile of Si in the sample c4 shown in Table 3.

FIG. 7C is a SIMS profile of Si in a sample c6 shown in Table 3.

FIG. 7D is a SIMS profile of Si in a sample c7 shown in Table 3.

FIG. 8A is a SIMS profile of Cl in the sample c3 shown in Table 3.

FIG. 8B is a SIMS profile of Cl in the sample c4 shown in Table 3.

FIG. 8C is a SIMS profile of Cl in the sample c6 shown in Table 3.

FIG. 8D is a SIMS profile of Cl in the sample c7 shown in Table 3.

FIG. 9 is a graph showing a relation between carrier density and electron mobility in the β-Ga₂O₃-based single crystal film.

DESCRIPTION OF EMBODIMENT Embodiment (Configuration of Crystal Laminate Structure)

FIG. 1 is a vertical cross-sectional view showing a crystal laminate structure 1 in an embodiment. The crystal laminate structure 1 has a Ga₂O₃-based substrate 10 and a β-Ga₂O₃-based single crystal film 12 formed on a principal surface 11 of the Ga₂O₃-based substrate 10 by epitaxial crystal growth.

The Ga₂O₃-based substrate 10 is a substrate formed of a Ga₂O₃-based single crystal with a β-crystal structure. The Ga₂O₃-based single crystal here means a Ga₂O₃ single crystal or is a Ga₂O₃ single crystal doped with an element such as Al or In, and may be, e.g., a (Ga_(x)Al_(y)In_((1-x-y)))₂O₃ (0<x≦1, 0≦y≦1, 0<x+y≦1) single crystal which is a Ga₂O₃ single crystal doped with Al and In. The band gap is widened by adding Al and is narrowed by adding In. The Ga₂O₃-based substrate 10 may also contain a conductive impurity such as Si.

The plane orientation of the principal surface 11 of the Ga₂O₃-based substrate 10 is, e.g., (001), (010), (−201) or (101).

To form the Ga₂O₃-based substrate 10, for example, a bulk crystal of a Ga₂O₃-based single crystal grown by, e.g., a melt-growth technique such as the FZ (Floating Zone) method or the EFG (Edge Defined Film Fed Growth) method is sliced and the surface thereof is then polished.

The β-Ga₂O₃-based single crystal film 12 is formed of a Ga₂O₃-based single crystal with a β-crystal structure in the same manner as the Ga₂O₃-based substrate 10. The β-Ga₂O₃-based single crystal film 12 also contains a dopant such as Li, Na, K, Rb, Cs, Be, Mg, Ca, Sr, Ba, Sc, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Re, Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au, Zn, Cd, Hg, B, Al, In, Tl, C, Si, Ge, Sn, Pb, N, P, As, Sb, Bi, S, Se or Te, etc., which is doped during crystal growth.

A concentration of the dopant contained in the β-Ga₂O₃-based single crystal film 12 is not less than 1×10¹³ atoms/cm³ and not more than 5×10²⁰ atoms/cm³, preferably, not less than 6.5×10¹⁵ atoms/cm³ and not more than 2.1×10²⁰ atoms/cm³.

A density of carrier generated by the doping of the dopant is, e.g., not less than 3.2×10¹⁵ cm⁻³ and not more than 1.8×10¹⁸ cm⁻³.

The β-Ga₂O₃-based single crystal film 12 also contains Cl at a concentration of not more than 5×10¹⁶ atoms/cm³. This results from that the β-Ga₂O₃-based single crystal film 12 is formed by the HVPE method using Cl-containing gas. Generally, Cl-containing gas is not used to form a β-Ga₂O₃-based single crystal film when using a method other than the HVPE method, and the β-Ga₂O₃-based single crystal film does not contain Cl, or at least does not contain 1×10¹⁶ cm⁻³ or more of Cl.

Meanwhile, the β-Ga₂O₃-based single crystal film 12 is formed by the HYPE (Halide Vapor Phase Epitaxy) method with a high crystal growth rate, and thus can be formed thick, e.g., can have a thickness of not less than 1000 nm. In general, a growth rate of the β-Ga₂O₃-based single crystal film by industrial HVPE is 200 μm/h, and in this case, a film of up to 1000 μm in thickness can be grown in a realistic production time. In other words, it is possible to form the β-Ga₂O₃-based single crystal film 12 having a thickness of not less than 1000 nm and not more than 1000 μm. In this regard, use of the MBE method is not realistic in actual production since a crystal growth rate of the β-Ga₂O₃-based single crystal film is about 120 nm/h and it requires not less than 8 hours to form a film of not less than 1000 nm in thickness.

Additionally, a buffer layer may be provided between the Ga₂O₃-based substrate 10 and the β-Ga₂O₃-based single crystal film 12 to prevent carrier compensation in the β-Ga₂O₃-based single crystal film 12 due to impurity diffusion from the Ga₂O₃-based substrate 10.

(Structure of Vapor Phase Deposition System)

A structure of a vapor phase deposition system used for growing the β-Ga₂O₃-based single crystal film 12 in the present embodiment will be described below as an example.

FIG. 2 is a vertical cross-sectional view showing a vapor phase deposition system 2 in the embodiment. The vapor phase deposition system 2 is a vapor phase deposition system using the HVPE method, and has a reaction chamber 20 having a first gas introducing port 21, a second gas introducing port 22, a third gas introducing port 23, a fourth gas introducing port 24 and an exhaust port 25, and a first heating means 27 and a second heating means 28 which are placed to surround the reaction chamber 20 to heat predetermined regions in the reaction chamber 20.

The growth rate when using the HVPE method is higher than that in the MBE method, etc. In addition, in-plane distribution of film thickness is highly uniform and it is possible to grow a large-diameter film. Therefore, it is suitable for mass production of crystal.

The reaction chamber 20 has a source reaction region R1 in which a reaction container 26 containing a Ga source is placed and a gallium source gas is produced, and a crystal growth region R2 in which the Ga₂O₃-based substrate 10 is placed and the β-Ga₂O₃-based single crystal film 12 is grown thereon. The reaction chamber 20 is formed of, e.g., quartz glass.

Here, the reaction container 26 is formed of, e.g., quartz glass and the Ga source contained in the reaction container 26 is metal gallium.

The first heating means 27 and the second heating means 28 are capable of respectively heating the source reaction region R1 and the crystal growth region R2 of the reaction chamber 20. The first heating means 27 and the second heating means 28 are, e.g., resistive heaters or radiation heaters.

The first gas introducing port 21 is a port for introducing a Cl-containing gas (Cl₂ gas or HCl gas) into the source reaction region R1 of the reaction chamber 20 using an inert carrier gas (N₂ gas, Ar gas or He gas). The second gas introducing port 22 is a port for introducing an oxygen-containing gas (O₂ gas or H₂O gas, etc.) as an oxygen source gas into the crystal growth region R2 of the reaction chamber 20 using an inert carrier gas (N₂ gas, Ar gas or He gas). The third gas introducing port 23 is a port for introducing an inert carrier gas (N₂ gas, Ar gas or He gas) into the crystal growth region R2 of the reaction chamber 20. The fourth gas introducing port 24 is a port for introducing a source gas of dopant to be added to the β-Ga₂O₃-based single crystal film 12, such as Si (e.g., silicon tetrachloride, etc.), into the crystal growth region R2 of the reaction chamber 20 using an inert carrier gas (N₂ gas, Ar gas or He gas).

(Growth of β-Ga₂O₃-Based Single Crystal Film)

To grow the β-Ga₂O₃-based single crystal film 12, it is possible to use a technique of growing β-Ga₂O₃-based single crystal film disclosed in Japanese Patent Application No. 2014-088589. A process of growing the β-Ga₂O₃-based single crystal film 12 in the present embodiment will be described below as an example.

Firstly, the source reaction region R1 of the reaction chamber 20 is heated by the first heating means 27 and an atmosphere temperature in the source reaction region R1 is then maintained at a predetermined temperature.

Next, in the source reaction region R1, a Cl-containing gas introduced through the first gas introducing port 21 using a carrier gas is reacted with the metal gallium in the reaction container 26 at the above-mentioned atmosphere temperature, thereby producing a gallium chloride gas.

The atmosphere temperature in the source reaction region R1 here is preferably a temperature at which GaCl gas has the highest partial pressure among component gases of the gallium chloride gas produced by the reaction of the metal gallium in the reaction container 26 with the Cl-containing gas. The gallium chloride gas here contains GaCl gas, GaCl₂ gas, GaCl₃ gas and (GaCl₃)₂ gas, etc.

The temperature at which a driving force for growth of Ga₂O₃ crystal is maintained is the highest with the GaCl gas among the gases contained in the gallium chloride gas. Growth at a high temperature is effective to obtain a high-quality Ga₂O₃ crystal with high purity. Therefore, for growing the β-Ga₂O₃-based single crystal film 12, it is preferable to produce a gallium chloride gas in which a partial pressure of GaCl gas having a high driving force for growth at a high temperature is high.

It is possible to increase a partial pressure ratio of the GaCl gas in the gallium chloride gas by reacting the metal gallium with the Cl-containing gas at an atmosphere temperature of about not less than 300° C. Therefore, it is preferable that the metal gallium in the reaction container 26 be reacted with the Cl-containing gas in a state that the atmosphere temperature in the source reaction region R1 is maintained at not less than 300° C. by using the first heating means 27.

At the atmosphere temperature of, e.g., 850° C., the partial pressure ratio of the GaCl gas is predominantly high (the equilibrium partial pressure of the GaCl gas is four orders of magnitude greater than the GaCl₂ gas and is eight orders of magnitude greater than the GaCl₃ gas) and the gases other than GaCl gas hardly contribute to the growth of Ga₂O₃ crystal.

Meanwhile, considering the lifetime of the first heating means 27 and heat resistance of the reaction chamber 20 formed of quartz glass, etc., it is preferable that the metal gallium in the reaction container 26 be reacted with the Cl-containing gas in a state that the atmosphere temperature in the source reaction region R1 is maintained at not more than 1000° C.

In addition, if hydrogen is contained in an atmosphere for growing the β-Ga₂O₃-based single crystal film 12, surface flatness and a driving force for growth of the β-Ga₂O₃-based single crystal film 12 decrease. Therefore, it is preferable that a Cl₂ gas not containing hydrogen be used as the Cl-containing gas.

Next, in the crystal growth region R2, the gallium chloride gas produced in the source reaction region R1 is mixed with the oxygen-containing gas introduced through the second gas introducing port 22 and the dopant source gas such as Si introduced through the fourth gas introducing port 24, the Ga₂O₃-based substrate 10 is then exposed to the mixed gas, and the β-Ga₂O₃-based single crystal film 12 containing the dopant is thereby epitaxially grown on the Ga₂O₃-based substrate 10. At this time, in a furnace housing the reaction chamber 20, pressure in the crystal growth region R2 is maintained at, e.g., 1 atm.

The dopant source gas used here is preferably a chloride-based gas to prevent other unintentional impurities from being mixed. For example, when Si, Ge, Sn or Pb (Group 14 element) is selected from the dopants listed above and is used as a dopant, a chloride-based gas such as SiCl₄, GeCl₄, SnCl₄ or PbCl₂, respectively, is used. Here, the chloride-based gas is not limited to a compound of an element with only chlorine, and may be, e.g., a silane-based gas such as SiHCl₃.

The dopant such as Si is doped during growth of a β-Ga₂O₃-based single crystal.

If hydrogen is contained in an atmosphere for growing the β-Ga₂O₃-based single crystal film 12, surface flatness and a driving force for growth of the β-Ga₂O₃-based single crystal film 12 decrease. Therefore, it is preferable that an O₂ gas not containing hydrogen be used as the oxygen-containing gas.

Meanwhile, the smaller the equilibrium partial pressure of the GaCl gas, the more the GaCl gas is consumed for growth of Ga₂O₃ crystal and the Ga₂O₃ crystal grows efficiently. For example, the equilibrium partial pressure of the GaCl gas sharply falls when a ratio of the supplied partial pressure of the O₂ gas to the supplied partial pressure of the GaCl gas (the O₂/GaCl supplied partial pressure ratio) is not less than 0.5. Therefore, to efficiently grow the β-Ga₂O₃-based single crystal film 12, the β-Ga₂O₃-based single crystal film 12 is preferably grown in a state that the O₂/GaCl supplied partial pressure ratio in the crystal growth region R2 is not less than 0.5.

FIG. 3 is a graph which is obtained by thermal equilibrium calculation and shows a relation between an equilibrium partial pressure of GaCl and an O₂/GaCl supplied partial pressure ratio when the atmosphere temperature during Ga₂O₃ crystal growth is 1000° C. It is calculated using the supplied partial pressure value of the GaCl gas fixed to 1×10⁻³ atm, a furnace pressure of 1 atom adjusted by using, e.g., an inert gas such as N₂ as a carrier gas, and various values of the O₂ gas supplied partial pressure.

In FIG. 3, the horizontal axis indicates the O₂/GaCl supplied partial pressure ratio and the vertical axis indicates an equilibrium partial pressure (atm) of GaCl gas. It is shown that the smaller the equilibrium partial pressure of the GaCl gas, the more the GaCl gas is consumed for growth of Ga₂O₃ crystal, i.e., the Ga₂O₃ crystal grows efficiently.

FIG. 3 shows that the equilibrium partial pressure of the GaCl gas sharply falls at the O₂/GaCl supplied partial pressure ratio of not less than 0.5.

Meanwhile, a growth temperature of not less than 900° C. is required to grow the β-Ga₂O₃-based single crystal film 12. A single crystal may not be obtained at less than 900° C.

(Evaluation of β-Ga₂O₃-Based Single Crystal Film)

Plural β-Ga₂O₃-based single crystal films 12 were formed under different growth conditions, and Si concentration and carrier density thereof were measured. The results are shown in Tables 1 to 3 below.

Samples a1 and a2 shown in Table 1 are the crystal laminate structures 1 each having the β-Ga₂O₃-based single crystal film 12 formed under the condition that a partial pressure of the GaCl gas supplied as a Ga source gas, P⁰ _(GaCl), was fixed to 2×10⁻⁴ atm. Meanwhile, samples b1 to b12 shown in Table 2 are the crystal laminate structures 1 each having the β-Ga₂O₃-based single crystal film 12 formed under the condition that P⁰ _(GaCl) was fixed to 5×10⁻⁴ atm, and samples c1 to c9 shown in Table 3 are the crystal laminate structures 1 each having the β-Ga₂O₃-based single crystal film 12 formed under the condition that P⁰ _(GaCl) was fixed to 1×10⁻³ atm.

In the samples a1, a2, b1 to b12 and c1 to c9, a 7 μm-thick Si-containing β-Ga₂O₃ single crystal film was formed as the β-Ga₂O₃-based single crystal film 12 on a β-Ga₂O₃ substrate having a (001)-oriented principal surface used as the Ga₂O₃-based substrate 10.

“R_(Si)” in Tables 1 to 3 is a physical quantity expressed by P⁰ _(SiCl4)/(P⁰ _(GaCl)+P⁰ _(SiCl4)). P⁰ _(SiCl4) here is a partial pressure of the SiCl₄ gas supplied as a Si source gas during growth of the β-Ga₂O₃-based single crystal film 12. The value of P⁰ _(GaCl) for forming the β-Ga₂O₃-based single crystal film 12 is not specifically limited as long as pressure in the crystal growth region R2 of the vapor phase deposition system 2 (e.g., 1 atm) is maintained.

“Si concentration” in Tables 1 to 3 is a concentration of Si as a dopant in the β-Ga₂O₃-based single crystal film 12 (in a region at a depth of 2 to 6 μm from the surface) obtained by SIMS (Secondary Ion Mass Spectrometry). The background level of the Si concentration measured by SIMS differs depending on the measurement conditions. The Si concentration of the samples b2 and b3 was measured under the condition that the background level was about 3.0×10¹⁵ atoms/cm³, the Si concentration of the other samples was measured under the condition that the background level was about 3.0×10¹⁶ atoms/cm³, and Si at a concentration of not more than the background level is not detectable in each sample.

“N_(d)−N_(a)” in Tables 1 to 3 is a difference between a donor concentration N_(d) and an acceptor concentration N_(a), i.e., a carrier concentration, obtained by ECV (Electrochemical Capacitance-Voltage) measurement.

TABLE 1 Sample Si concentration N_(d) − N_(a) number R_(Si) [atoms/cm³] [cm⁻³] a1 6.3 × 10⁻⁶ 7.1 × 10¹⁷ — a2 3.1 × 10⁻⁵ 2.1 × 10¹⁸ —

TABLE 2 Sample Si concentration N_(d) − N_(a) number R_(Si) [atoms/cm³] [cm⁻³] b1 2.5 × 10⁻⁸ — 7.4 × 10¹⁵ b2 2.5 × 10⁻⁸ 6.5 × 10¹⁵ — b3 5.0 × 10⁻⁸ 1.0 × 10¹⁶ — b4 2.5 × 10⁻⁷ — 3.6 × 10¹⁶ b5 2.5 × 10⁻⁶ 1.7 × 10¹⁷ — b6 2.5 × 10⁻⁶ 2.2 × 10¹⁷ — b7 5.0 × 10⁻⁶ 3.5 × 10¹⁷ 2.8 × 10¹⁷ b8 1.0 × 10⁻⁵ 5.7 × 10¹⁷ 5.7 × 10¹⁷ b9 1.3 × 10⁻⁵ 5.3 × 10¹⁷ 5.2 × 10¹⁷  b10 1.3 × 10⁻⁵ 5.7 × 10¹⁷ —  b11 6.3 × 10⁻⁵ 2.7 × 10¹⁸ —  b12 7.5 × 10⁻⁴ 1.4 × 10²⁰ —

TABLE 3 Sample Si concentration N_(d) − N_(a) number R_(Si) [atoms/cm³] [cm⁻³] c1 1.3 × 10⁻⁸ — 3.4 × 10¹⁵ c2 1.3 × 10⁻⁷ — 2.0 × 10¹⁶ c3 1.3 × 10⁻⁶ 8.8 × 10¹⁶ — c4 2.5 × 10⁻⁶ 1.5 × 10¹⁷ — c5 3.1 × 10⁻⁶ 1.2 × 10¹⁷ 1.9 × 10¹⁷ c6 6.3 × 10⁻⁶ 2.2 × 10¹⁷ — c7 1.3 × 10⁻⁵ 4.0 × 10¹⁷ — c8 3.1 × 10⁻⁵ 1.2 × 10¹⁸ 1.8 × 10¹⁸ c9 1.9 × 10⁻³ 2.1 × 10²⁰ —

From Tables 2 and 3, it is understood that the carrier density obtained by ECV measurement substantially coincides with the Si concentration obtained by SIMS measurement. From Table 4 which is described later, it is understood that the carrier density obtained by Hall measurement also substantially coincides with the Si concentration obtained by SIMS measurement.

FIG. 4 is a graph which is obtained from the data of Tables 1 to 3 and Table 4 (described later) and shows a relation between R_(Si) and Si concentration of the grown β-Ga₂O₃-based single crystal film 12.

In FIG. 4, plot symbols “⋄” indicate the measured values for the samples a1 and a2 shown in Table 1, “◯” indicate the measured values for the samples b5 to b12 shown in Table 2, and “□” indicate the measured values for the samples c3 to c9 shown in Table 3. Plot symbols “●” in FIG. 4 indicate the measured values for the samples d3 and d4 shown in Table 4.

FIG. 4 shows that the relation between R_(Si) and the Si concentration of the β-Ga₂O₃-based single crystal film 12 is a substantially linear relation and the Si concentration of the β-Ga₂O₃-based single crystal film 12 can be controlled by adjusting the SiCl₄ gas supply amount relative to the GaCl gas supply amount.

FIG. 5 is a graph showing a relation between R_(Si) and carrier density of the grown β-Ga₂O₃-based single crystal film 12.

In FIG. 5, plot symbols “◯” indicate the measured values for the samples b1, b4 and b7 to b9 shown in Table 2, and “□” indicate the measured values for the samples c1, c2, c5 and c8 shown in Table 3. Plot symbols “●” in FIG. 5 indicate the measured values for the samples d1 to d4 shown in Table 4 (described later).

FIG. 5 shows that the relation between R_(Si) and the carrier density of the β-Ga₂O₃-based single crystal film 12 is a substantially linear relation.

From the actual measurement data shown in FIG. 4, it is understood that it is possible to control the Si concentration at least in the range of not less than 6.5×10¹⁵ atoms/cm³ and not more than 2.1×10²⁰ atoms/cm³. Furthermore, based on the linearity of the actual measurement data shown in FIG. 4, it is considered that the Si concentration can be controlled in a wider range.

However, when trying to dope Si at a concentration of less than about 1×10¹³ atoms/cm³, it is difficult to accurately control the partial pressure of the doping gas and it is thus difficult to control the Si concentration. Meanwhile, the upper limit of the Si concentration at which crystal quality of the β-Ga₂O₃-based single crystal film 12 can be maintained by solid solubility of Si in β-Ga₂O₃-based single crystal is about 5.0×10²⁰ atoms/cm³. Thus, it is preferable to control the Si concentration in the range of not less than 1×10¹³ atoms/cm³ and not more than 5.0×10²⁰ atoms/cm³.

Meanwhile, from the actual measurement data shown in FIG. 5, it is understood that it is possible to control the carrier density at least in the range of not less than 3.2×10¹⁵ cm⁻³ and not more than 1.8×10¹⁸ cm⁻³ by adding the dopant. Furthermore, based on the linearity of the actual measurement data shown in FIG. 5, it is considered that the carrier density can be controlled in a wider range, in the same manner as the Si concentration.

In an undoped β-Ga₂O₃-based single crystal film grown by the HYPE method, carriers with a density of not more than 1×10¹³ cm⁻³ remains as disclosed in Japanese Patent Application No. 2014-088589, and it is difficult to control the carrier concentration to be lower than 1×10¹³ cm⁻³. In addition, the upper limit of the Si concentration at which crystal quality of the β-Ga₂O₃-based single crystal film 12 can be maintained is about 5.0×10²⁰ atoms/cm³ as described above. Therefore, for the carrier concentration which substantially coincides with the Si concentration, the upper limit at which crystal quality of the β-Ga₂O₃-based single crystal film 12 can be maintained is considered to be about 5.0×10²⁰ cm⁻³. Thus, it is preferable to control the carrier density of the Ga₂O₃-based single crystal film in the range of not less than 1×10¹³ cm⁻³ and not more than 5.0×10²⁰ cm⁻³.

FIG. 6 is a SIMS profile of Sn in the sample c4 shown in Table 3. In FIG. 6, the horizontal axis indicates a depth from the surface of the crystal laminate structure 1 (from the surface of the β-Ga₂O₃-based single crystal film 12), and the vertical axis indicates Sn concentration. In addition, a dotted line in FIG. 6 indicates the background level (2.0×10¹⁷ atoms/cm³) of the Sn concentration.

Sn is a dopant contained in the Ga₂O₃-based substrate 10 of the sample c4. From the SIMS profile of Sn in FIG. 6, it is understood that an interface between the Ga₂O₃-based substrate 10 and the β-Ga₂O₃-based single crystal film 12 is located at a depth about of 7 μm from the surface of the crystal laminate structure 1. Since the β-Ga₂O₃-based single crystal films 12 of all samples shown in Table 1 to 3 were formed under the same growth conditions, an interface between the Ga₂O₃-based substrate 10 and the β-Ga₂O₃-based single crystal film 12 in each sample is located at a depth of about 7 μm from the surface of the crystal laminate structure 1.

FIGS. 7A to 7D are SIMS profiles of Si in the samples c3, c4, c6 and c7 shown in Table 3. In FIGS. 7A to 7D, the horizontal axis indicates a depth from the surface of the crystal laminate structure 1, and the vertical axis indicates Si concentration. In addition, in FIGS. 7A to 7D, a horizontal dotted line indicates the background level of the Si concentration (3.0×10¹⁶ atoms/cm³) and a vertical dot-and-dash line indicates the position of the interface between the Ga₂O₃-based substrate 10 and the β-Ga₂O₃-based single crystal film 12 (depth of 7 μm).

FIGS. 8A to 8D are SIMS profiles of Cl in the samples c3, c4, c6 and c7 shown in Table 3. In FIGS. 8A to 8D, the horizontal axis indicates a depth from the surface of the crystal laminate structure 1, and the vertical axis indicates Cl concentration. In addition, in FIGS. 8A to 8D, a horizontal dotted line indicates the background level of the Cl concentration (2.0×10¹⁵ atoms/cm³) and a vertical dot-and-dash line indicates the position of the interface between the Ga₂O₃-based substrate 10 and the β-Ga₂O₃-based single crystal film 12 (depth of 7 μm).

According to FIGS. 8A to 8D, the β-Ga₂O₃ single crystal film contains Cl at a concentration of not more than about 5×10¹⁶ atoms/cm³. This results from that the Ga₂O₃ single crystal film is formed by the HVPE method using Cl-containing gas. Generally, Cl-containing gas is not used to form a Ga₂O₃ single crystal film when using a method other than the HVPE method, and the Ga₂O₃ single crystal film does not contain Cl, or at least does not contain 1×10¹⁶ atoms/cm³ or more of Cl.

Plural β-Ga₂O₃ single crystal films were formed under different growth conditions, and carrier density, resistivity and electron mobility thereof were measured. The results are shown in Table 4 below. These values were obtained by Hall measurement using the Van del Pauw method. In the Hall measurement, a DC magnetic field of 0.57 T and an electric current of 0.1 mA were applied at room temperature (25° C.). Table 4 also shows the Si concentration in the β-Ga₂O₃-based single crystal film (in a region at a depth of 2 to 6 μm from the surface) measured by SIMS.

The samples d1 to d4 to be subjected to Hall measurement were made as follows. Firstly, a β-Ga₂O₃ single crystal film was epitaxially grown on a highly Fe-doped β-Ga₂O₃ substrate having a (001)-oriented principal surface. At this time, SiCl₄ gas supply was started in the middle of the growth of the β-Ga₂O₃ single crystal film so that a 2 μm-thick undoped β-Ga₂O₃ single crystal film and an 8 μm-thick Si-doped β-Ga₂O₃ single crystal film were formed. The undoped β-Ga₂O₃ single crystal film has a function of preventing diffusion of Fe from the β-Ga₂O₃ substrate and thus preventing carrier compensation in the Si-doped β-Ga₂O₃ single crystal film.

The β-Ga₂O₃ single crystal film here was formed under the condition that P⁰ _(GaCl) was fixed to 1×10⁻³ atm.

Next, for surface flattening, the surface of the Si-doped β-Ga₂O₃ single crystal film was polished 2 μm by CMP (Chemical Mechanical Polishing). Next, circular In electrodes having a diameter of 1 mm were formed at four corners on the surface of the Si-doped β-Ga₂O₃ single crystal film and were then annealed in a N₂ atmosphere at 900° C. for 90 seconds, thereby forming ohmic electrodes.

In the samples d1 to d4 to be subjected to Hall measurement, the β-Ga₂O₃ substrate corresponds to the Ga₂O₃-based substrate 10 and the Si-doped β-Ga₂O₃ single crystal film corresponds to the β-Ga₂O₃-based single crystal film 12.

TABLE 4 Sam- Si ple concen- num- N_(d) − N_(a) Resistivity Mobility tration ber R_(Si) [cm⁻³] [Ω · cm] [cm²/V · s] [atoms/cm³] d1 1.3 × 10⁻⁸ 3.2 × 10¹⁵ 12.5 1.60 × 10² — d2 1.3 × 10⁻⁷ 1.2 × 10¹⁶ 3.44 1.55 × 10² — d3 3.1 × 10⁻⁶ 2.2 × 10¹⁷ 2.47 × 10⁻¹ 1.14 × 10² 2.0 × 10¹⁷ d4 3.1 × 10⁻⁵ 1.2 × 10¹⁸ 5.96 × 10⁻² 8.98 × 10  1.1 × 10¹⁸

FIG. 9 is a graph which is obtained from the data of Table 4 and shows a relation between carrier density (N_(d)−N_(a)) and electron mobility in the β-Ga₂O₃-based single crystal film 12.

FIG. 9 shows that electron mobility increases with a decrease in the carrier density of the β-Ga₂O₃-based single crystal film 12.

Although the β-Ga₂O₃ substrate having a (001)-oriented principal surface was used as the Ga₂O₃-based substrate 10 in the samples a1, a2, b1 to b12, c1 to c9 and d1 to d4 as described above, similar evaluation results are obtained also when a β-Ga₂O₃ substrate having a (−201)-, (101)- or (010)-oriented, principal surface is used. In addition, similar evaluation results are obtained also when another β-Ga₂O₃-based substrate is used instead of the β-Ga₂O₃ substrate or when another β-Ga₂O₃-based single crystal film is formed instead of the β-Ga₂O₃ single crystal film.

Effects of the Embodiment

In the embodiment, by adding a dopant while growing a β-Ga₂O₃-based single crystal using the HVPE method, it is possible to control the dopant concentration of the β-Ga₂O₃-based single crystal in a wider range than when using the MBE method or the EFG method. In addition, unlike when using ion implantation, problems such as limitation of dopant implantation depth or deterioration of crystallinity due to ion beam do not occur.

Although the embodiment of the invention has been described, the invention is not intended to be limited to the embodiment, and the various kinds of modifications can be implemented without departing from the gist of the invention.

In addition, the invention according to claims is not to be limited to the embodiment described above. Further, it should be noted that all combinations of the features described in the embodiment are not necessary to solve the problem of the invention.

INDUSTRIAL APPLICABILITY

Provided is a crystal laminate structure having a β-Ga₂O₃-based single crystal film in which a dopant is included throughout the crystal and the concentration of the dopant can be set across a broad range.

REFERENCE SIGNS LIST

-   1 CRYSTAL LAMINATE STRUCTURE -   10 Ga₂O₃-BASED SUBSTRATE -   11 PRINCIPAL SURFACE -   12 β-Ga₂O₃-BASED SINGLE CRYSTAL FILM 

1. A crystal laminate structure, comprising: a Ga₂O₃-based substrate; and a β-Ga₂O₃-based single crystal film formed by epitaxial crystal growth on a principal surface of the Ga₂O₃-based substrate, wherein the β-Ga₂O₃-based single crystal film comprises Cl and a dopant doped in parallel with the crystal growth at a concentration of not less than 1×10¹³ atoms/cm³ and not more than 5.0×10²⁰ atoms/cm³.
 2. The crystal laminate structure according to claim 1, wherein a concentration of the Cl of the β-Ga₂O₃-based single crystal film is not more than 5×10¹⁶ atoms/cm³.
 3. The crystal laminate structure according to claim 1, wherein the dopant comprises Si.
 4. The crystal laminate structure according to claim 1, wherein a concentration of the dopant is not less than 6.5×10¹⁵ atoms/cm³ and not more than 2.1×10²⁰ atoms/cm³.
 5. The crystal laminate structure according to claim 1, wherein a density of a carrier generated by the doping of the dopant is not less than 1×10¹³ cm⁻³ and not more than 5.0×10²⁰ cm⁻³.
 6. The crystal laminate structure according to claim 5, wherein a density of a carrier generated by the doping of the dopant is not less than 3.2×10¹⁵ cm⁻³ and not more than 1.8×10¹⁸ cm⁻³.
 7. The crystal laminate structure according to claim 1, wherein the β-Ga₂O₃-based single crystal film comprises a β-Ga₂O₃ crystal film.
 8. The crystal laminate structure according to claim 1, wherein the principal surface of the Ga₂O₃-based substrate has a plane orientation of (001), (010), (−201) or (101).
 9. The crystal laminate structure according to claim 3, wherein a source gas of the dopant comprises a SiCl₄ gas.
 10. The crystal laminate structure according to claim 1, wherein a thickness of the β-Ga₂O₃-based single crystal film is not less than 1000 nm.
 11. The crystal laminate structure according to claim 2, wherein the dopant comprises Si.
 12. The crystal laminate structure according to claim 2, wherein a concentration of the dopant is not less than 6.5×10¹⁵ atoms/cm³ and not more than 2.1×10²⁰ atoms/cm³.
 13. The crystal laminate structure according to claim 2, wherein a density of a carrier generated by the doping of the dopant is not less than 1×10¹³ cm⁻³ and not more than 5.0×10²⁰ cm⁻³.
 14. The crystal laminate structure according to claim 13, wherein a density of a carrier generated by the doping of the dopant is not less than 3.2×10¹⁵ cm⁻³ and not more than 1.8×10¹⁸ cm⁻³.
 15. The crystal laminate structure according to claim 2, wherein the β-Ga₂O₃-based single crystal film comprises a β-Ga₂O₃ crystal film.
 16. The crystal laminate structure according to claim 2, wherein the principal surface of the Ga₂O₃-based substrate has a plane orientation of (001), (010), (−201) or (101).
 17. The crystal laminate structure according to claim 11, wherein a source gas of the dopant comprises a SiCl₄ gas.
 18. The crystal laminate structure according to claim 2, wherein a thickness of the β-Ga₂O₃-based single crystal film is not less than 1000 nm. 